1. Field of Invention
This invention relates to improved molding apparatuses and more particularly molding apparatuses used for producing centrally-apertured discs (e.g., record discs, video disks, magnetic discs, optical discs and information recording discs). The molding apparatuses according to the present invention include non-compression and compression injection molding apparatuses.
2. Reference to Related Art
U.S. Pat. No. 4,085,178 relates to a method for producing injection molded and centrally apertured disc records. In this patent, after partial cooling of the heated injected material, the first and second mold halves move from the closed location to a location intermediate a closed location to an open location while the punch is locked in place to sever the sprue from the part along the peripheral surface of the punch end portion. Furthermore, the sprue bushing displaced opposing the punch is secured to a platen which is secured to a frame of an injection molding machine, so that the sprue bushing is arranged to be immovable. The application of U.S. Pat. No. 4,085,178 is a divisional application of Ser. No. 642,162 (now U.S. Pat. No, 3,989,436) and the machine used in the method of U.S. Pat. No. 4,085,178 has the same structure with that of the parent application.
In an injection molding machine described in U.S. Pat. No. 4,260,360 and U.S. Pat. No. 4,185,955, a punch is locked in place during which first and second mold halves held by a latching device move from the closed location to an intermediate location, at which position a sprue is fully severed from the part of disc by the peripheral surface of the punch end portion. In these patents, an immovable sprue bushing is provided.
Japanese Patent Laid-open No. 59-230,731 shows an injection molding machine having such a structure that when an aperture is formed at the center of a disc, only a sprue bushing is retreated, at a non-load state, with the advancement of a punch at a state that the mold halves are closed.
In centrally apertured discs such as record discs, magnetic discs, optical discs and information recording discs, particularly optical discs of this type, as conditions to obtain reproduction of clear image and high quality sound, it is required to have low birefringence and have no difference of birefringence between the outer and inner peripheries of the discs (i.e., have a uniform birefringence).
All of the above-listed related art have common structure such that no resistance is applied to a punch when a central aperture is severed by the punch. However, under such structures, a cutting face of the central aperture of the molded disc becomes coarse and the central portion of the molded disc is flexed at the time of the punching, so that residual stress is generated. Thus, there arises the problem that the accurate signal encoding range around the aperture is reduced.
The present invention is provided to solve the above-mentioned problems and improves the conditions required.
In the Japanese related art molding apparatus noted above, a punch is provided to form a central aperture in the molded disc, and the molded disc is punched to form the central aperture with advancement of the punch. This operation is shown in the time chart of FIG. 10. In the Figure, a line X1 shows an ordinary injection molding operation. As is shown, the operation consists of an injection step I, a cooling step II, a gas release by depressurization step III, a low speed mold opening step IV, a high speed mold opening step V, a low speed mold opening step VI, a product ejector pin advancement step VII and an ejector pin retreat step VIII, these steps being executed in the mentioned order.
Further, upon completion of the injection step I, a charging process is started with the start of rotation of a screw fitted in a heating cylinder, as shown by a line X2. As is shown, the charging process consists of a feed step IX of supplying resin to the heating cylinder, a plasticizing step X and a suck-back step XI. These steps are being executed in the mentioned order. After the resin has been cooled down, the contact force of a nozzle provided at the end of the heating cylinder with respect to a movable sprue bushing in the molder is reduced to zero, and the punch is advanced to punch the molded disc to form a central hole. The punch is retreated in co-operation with the nozzle. The time before the retreat of the nozzle is labeled T1 in line X3. Then, the nozzle retreat step XII is started. The punch advancement step XIII is started after the lapse of time T2 equal to 0.5 sec., and the punch retreat step XIV is started after the lapse of a predetermined period T3 of time. The nozzle is stopped when it strikes a limit switch LS-N.
In the above related art molding apparatus, the punch is advanced after the resin in the molder cavity has been solidified. At this time, the central portion of the molded disc in the molder cavity is hardened. Therefore, a coarse cutting surface results when the central aperture is formed by the punch. On addition, cutting dust generated at this time becomes attached to a mirror surface of the molder cavity and also to the stamper. This will lead to the formation of a defective molding due to introduction of the cutting dust into the molten resin injected at the time of the next injection.